Granulomatosis with Polyangiitis, also known as Wegener's Granulomatosis or just Wegener's is a rare and deadly form of vasculitis. It is distinguished from other forms of vasulitis by the presence of granulomas and certain blood tests. The most determinant blood test is the C-ANCA. Wegener's is a frequent differential on the show, but it only appeared at the end of Season 7, where is was almost mistaken for cancer. This was not a stretch as both conditions respond well to chemo therapy and the granulomas can be mistaken for tumors, particularly where they cannot be biopsied.